The present invention is related to zinc alloys yielding anticorrosive coatings on ferrous materials, consisting of zinc, plus its usual impurities and possibly aluminium or lead together with alloying metals: nickel as well as vanadium and/or chrome.
Corrosion is a frequent but undesirable process in certain metals. To avoid corrosion the metals are usually coated with a layer of zinc.
There are different methods known and used to coat steel and other metals with zinc and zinc alloys, such as: hot dip galvanising, zinc spraying, etc. One of the oldest methods still in use for economical and technical reasons is the so-called hot dip galvanising process.
Hot dip galvanising basically consists of the immersion, for a few minutes, of ferrous materials in a molten zinc bath at a temperature of between 430 and 560xc2x0 C.
Hot dip immersion produces a physicochemical mechanism by which a diffusion process takes place between the base iron of the parts and the zinc.
The zinc coating gives the necessary good corrosion resistance to ferrous metals.
In general, a zinc coating obtained by hot dip galvanising consists of several layers: an internal alloy of iron and zinc which adheres to the surface of the ferrous material, and an external layer, consisting almost entirely of pure zinc, according to the composition of the bath, called the Eta phase. In the interior layer, formed by the diffusion of zinc into the ferrous material, up to three zones or sub-layers can be distinguished, identified by their different iron contents. The sub-layer closest to the base material is called the Gamma phase and contains 21 to 28% iron. Next is the Delta phase, which contains from 6% to 11% iron, and finally the Zeta phase which contains approximately 6% iron.
Depending on the composition of the ferrous material of the part to be coated, the Zeta phase varies greatly in thickness and often tends to pass through to the external layer consisting mainly of pure zinc.
When e.g. construction grade steel is galvanized in a conventional zinc bath, without additional alloying metals, a galvanised coating with a relatively thin Delta phase and a Zeta layer are produced. The Zeta layer consists of large column crystals and reaches out to very near to the surface of the coating, while the Eta layer of pure zinc is almost non-existent.
The resulting coating layer has very low adherence because of the thick iron rich Zeta phase.
PATENT ABSTRACTS OF JAPAN, vol. 096, no. 007, Jul. 13, 1996 and JP 08 060329 A (KOBE STEEL LTD) concerns the production of galvannealed steel sheet in a continuous hot-dip process wherein the zinc coating bath contains Al, as well as Ni, Co and/or Ti.
PATENT ABSTRACTS OF JAPAN, vol. 018, no. 052 (C-1158), Jan. 27, 1994 and JP 05 271892 A (NISSHIN STEEL CO. LTD) , describes a method for controlling galvanising bath. The aim of this invention is to reduce the influence of aluminium on the zinc bath in continuous hot-dip galvanising of steel sheet by the Ni addition. The coating bath contains Zn, Al and Ni.
PATENT ABSTRACTS OF JAPAN, vol. 017, no. 345 (C-1077), Jun. 30, 1993 and JP 05 044006 A (NIPPON STEEL CORP) is related to the production of alloyed hot-dip galvanising steel sheet having excellent workability and corrosion resistance. The galvanising bath contains Al and V.
PATENT ABSTRACTS OF JAPAN, vol. 017, no. 678 (C-1141), Dec. 13, 1993 and JP 05 222502 A (KAWASAKI STEEL CORP) concerns Znxe2x80x94Crxe2x80x94Al series hot-dip galvanised steel excellent in corrosion and peeling resistance and its manufacture. The goal of this invention is to obtain hot-dip galvanised steel using Znxe2x80x94Crxe2x80x94Al alloy with an excellent corrosion and peeling off resistance. On the surface of the steel to be galvanized is previously deposited a substance containing phosphorous.
PATENT ABSTRACTS OF JAPAN, vol. 016, no. 168 (C-0932) , Apr. 22, 1992 and JP 04 013856 A (NIPPON STEEL CORP), describes the production of galvannealed steel sheet having a superior corrosion resistance in a continuous hot-dip. The galvanising bath consists in a Znxe2x80x94Alxe2x80x94Cr alloy and includes a subsequent heat treatment at about 510xc2x0 C.
PATENT ABSTRACTS OF JAPAN, vol. 018, no. 114 (C-1171) , Feb. 24, 1994 and JP 05 306445 A (NIPPON STEEL CORP) is related to the manufacture of P-containing high strength galvannealed steel sheet. The phosphorous content is 0.01-0.2% and the composition of the bath is zinc, aluminium and one or-two of the following elements: Mn, Mg, Ca, Ti, V, Cr, Co and Ce.
The document GB 1 493 224 A (ITALSIDER SPA) concerns a zinc-based alloy of continuous coating of wire and steel sheet using the Sendzimir technique. The coating bath consists in Zn, Al, Mg, Cr, Ti.
The document EP 0 042 636 A (CENTRE RECHERCHE METALLURGIQUE) is about a process characterized by the use of a coating bath containing zinc with the addition of one or two of the following elements: Al, Be, Ce, Cr, La, Mg, Mn, Pb, Sb, Si, Sn, Ta, Ti, Te and Th to obtain over the first coating a supplementary protection layer formed by stable compounds.
None of these documents suggest the use of nickel together with vanadium and/or chrome as alloying metals for zinc.
The aims of the invention are to provide improved zinc base alloys used to coat parts made of ferrous material having a superior corrosion resistance.
Surprisingly, it was found that these aims could be achieved by means of specific alloying metals, more particularly by means of zinc alloy yielding anti-corrosive coatings on ferrous materials characterized as consisting of zinc plus its usual impurities and possibly aluminium and/or lead as well as alloying metals consisting of between x and y% of nickel together with between v and w% of at least one of the metals: vanadium and chrome wherein:
x is equal to or higher than 0.001, preferably higher than 0.04,
y is lower than or equal to 0.6, preferably lower than 0.2,
v is equal to or higher than 0.001, preferably higher than 0.03,
w is lower than or equal to 0.6, preferably lower than 0.04.
All the indicated percentages are expressed as % w/w throughout the specification and claims.
Without being bound by the explanations given, Applicants have observed that the use of these alloys produces a much thinner Zeta layer, resulting in an improvement of its mechanical resistance, and a relatively much thicker Eta layer, resulting in an important increase in the corrosion resistance of the coating. Vanadium giving generally better results than chrome is also usually preferred.